For example, an ink-jet printing device is known as one of image forming devices including a printer, a facsimile, a copier and a multi-function peripheral. The ink-jet printing device uses as a printing head a liquid discharge head to discharge an ink drop, as a droplet of printing liquid, to a recording sheet so that an image is formed on the recording sheet. The material of the recording sheet is not limited to paper, and the term “recording sheet” means that it contains what is called a recording medium, such as a copy sheet, copy paper, etc. Image formation, recording, printing, etc. are synonyms, and this image formation means that it includes not only formation of an image of characters, figures, etc. but also formation of an image of patterns or the like.
As a type of image forming device, a serial type image forming device is known. See Japanese Laid-Open Patent Application No. 2004-122681 and Japanese Laid-Open Patent Application No. 2004-122685. In the serial type image forming device, the printing head having a nozzle sequence with a plurality of nozzles for discharging a droplet of printing liquid is provided in the carriage, and a main scanning of the carriage over the sheet is performed while the sheet is fed intermittently in a direction perpendicular to the main scanning direction by a line feed amount that is less than a length of the nozzle sequence, so that an image is formed on the sheet.
When continuous printing in which images are formed continuously and sequentially to the plurality of sheets is performed with the serial type image forming device, if the sheet feeding is carried out such that the preceding sheet is fed at a time by a distance larger than the feed amount (the amount of line feed) at the time of image formation after the image formation to the preceding sheet is completed, the preceding sheet is ejected and the front edge of the following sheet is transported to the printing position, then the throughput of printing will fall.
To eliminate the problem, there is known a printing device in which printing to the rear edge of the preceding sheet and the front edge of the following sheet is carried out by a same main scanning while the plurality of sheets are transported. See Japanese Laid-Open Patent Application No. 2000-127368.
In the above-mentioned serial type image forming device, in order to shorten the print output time, the transporting speed of ejection operation of the preceding sheet and feeding operation of the following sheet is set up to a high speed as much as possible. For this reason, the sheet position may be made unstable and the sheet gap may become long. Since highly precise line feed transporting (line feed at the time of image formation) and high-speed transporting are mixed, it may induce an unnecessary vibration to the image forming device, the print dot position may be in disorder, and the image quality may deteriorate.
The scanning speed of the carriage depends on the pixel density and the drive frequency of the printing head that is decided according to the setting of dot formation. If the speed of the sheet feeding at the time of image formation is increased, in order to increase the printing speed when performing the continuous printing to the plurality of sheets, the accuracy of sheet stop position deteriorates, and there is a problem that the dot position is in disorder and the image quality deteriorates.
To eliminate the problem, the printing device of Japanese Laid-Open Patent Application No. 2000-127368 is aimed at improving the printing speed when the continuous printing is performed, such that printing to the rear edge of the preceding sheet and the front edge of the following sheet is carried out by a same main scanning and the sheet transporting is performed with a reduced gap between the preceding sheet and the following sheet.
However, even in the above-mentioned image forming device, there still is a problem that it is not is suitable for a case in which the quality of an image formed on the preceding sheet differs from the quality of an image formed on the following sheet. In addition, it is not is suitable for a case in which the timing to receive the data of an image formed on the following sheet delays.